1. Field of the Invention
The present invention relates to matching circuits to be used in amplifiers and the like and to power amplifiers. More specifically, the present invention relates to a multiband matching circuit that can establish matching between the input-output impedance of a circuit element having frequency dependence such as an amplification device and the impedance of a peripheral circuit in a plurality of frequency bands simultaneously and to a multiband power amplifier having the multiband matching circuit.
2. Description of the Related Art
As radio communication services have become more diverse in recent years, radio equipment is required to have a capability of dealing with signals in a plurality of frequency bands (multiband). In IEEE802.11/a/b/g wireless LAN standards, for example, a 5.2-GHz band and a 2.4-GHz band are specified.
Radio equipment contains a power amplifier for amplifying a signal in the radio frequency band and supplying the signal to an antenna. Since the power amplifier consumes much power, the operation efficiency should be improved. This requirement can be satisfied by providing a matching circuit optimized to improve the efficiency in the operating frequency band. A configuration that would be generally provided to optimize efficiency in two different frequency bands uses a switch for selecting a circuit optimally designed for each frequency band.
FIG. 13 shows the structure of a dual-band power amplifier 300 that can amplify signals in two frequency bands, as disclosed in Koji Chiba, et al. “Mobile Terminals,” NTT DoCoMo Technical Journal, 2002, Vol. 10, No. 1, pp. 15-20, and other documents. In the structure, a 5.2-GHz amplifier 10 designed specifically for the 5.2-GHz band or a 2.4-GHz amplifier 20 designed specifically for the 2.4-GHz is selected by two single-pole double-throw (SPDT) switches 30 in accordance with the operating frequency.
Both the 5.2-GHz amplifier 10 and the 2.4-GHz amplifier 20 in the dual-band power amplifier 300 shown in FIG. 13 include an input matching circuit 41, an amplification device 60, and an output matching circuit 42, as shown in FIG. 14. The performance of the amplifier is determined by the characteristics of the amplification device and the matching circuits. In the amplifiers 10 and 20, the amplification device 60 is connected between the matching circuits 41 and 42, which are optimized to establish matching in the corresponding frequency bands. In the dual-band power amplifier 300 shown in FIG. 13, the amplifiers 10 and 20 that include the optimized matching circuits are selected by the SPDT switches 30 in accordance with the operating frequency band. If the SPDT switches have sufficiently small insertion loss, each amplifier would perform high-output, high-efficiency operation.